The purpose of a catalytic converter is to convert pollutant materials in engine or turbine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxided, etc., to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square, circular, or triangular straight-through openings or cells, with catalyst deposited on the walls of the cells, catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads, or a corrugated thin metal foil monolith, e.g., a ferritic stainless steel foil having a catalyst carried on or supported by the surface. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere.
However, conversion is not efficient initially when the exhaust gases are relatively cold. To be effective at a high conversion rate, the catalyst and the surfaces of the converter with which the gases come in contact must be at a minimum temperature, e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC) and 1000 F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has come to its operating temperature, the catalytic converter is optimally effective. Hence, it is necessary to contact relatively cold exhaust gases with hot catalyst to effect satisfactory conversion at engine start-up. Both compression ignited (diesel) and spark ignited engines have this need.
To achieve initial heating of the catalyst prior to engine start-up, there is provided an electrically heatable catalytic converter formed of a corrugated thin metal monolith which is connected to a voltage source, e.g., a 12 volt to 108 volt automotive battery, and power supplied, preferably before, during, and possibly after start-up, to elevate and maintain the temperature of the catalyst to at least 650 F. plus or minus 20 F.
Copending application Ser. No. 587,219 filed Sep. 24, 1990 and its parent case Ser. No. 524,284 filed Apr. 16, 1990, disclose one form of electrically heatable catalytic converter which has been found to be subject to telescoping of the core, and each provides a means for offsetting the tendency to telescoping of the core in operation and ultimate destruction thereof under Hot Shake automotive tests. The disclosure of Ser. No. 587,219 is incorporated herein by reference thereto. Reference may also be had to Ser. No. 626,672 filed Dec. 12, 1990 which discloses a further improvement in resistance to telescoping by brazing between contiguous leaves whereby the leaves or convolutions are held against telescoping or axial displacement. The foregoing applications are commonly owned with the present application.
Copending application Ser. No. 606,130 filed Oct. 31, 1990 by William A. Whittenberger and entitled Electrically Heatable Catalytic Converter, and commonly owned with the present application, discloses a means for preventing telescoping of a spiral or S-wound corrugated thin metal foil monolith by brazing between the corrugated leaves according to a pattern (staggered relation). In that application, all of the leaves forming the monolith are corrugated. The adjacent leaves or strips are in nonnesting relationship by virtue of (1) pattern corrugations, such as herringbone corrugations, or (2) straight-through corrugations using corrugated leaves with straight cells of differing pitch. (See U.S. Pat. No. 4,810,588 dated Mar. 7, 1989 to Bullock)
This application also comtemplates the use of a collimator device in combination with reinforced foils such as disclosed in co-pending application Ser. No. 696,132 filed May 6, 1991, commonly owned with the present application. In Ser. No. 696,132, at least one longitudinal edge, optionally both longitudinal edges, of the foil is folded over through 180 degrees a distance of from 5% to 25% of the final width of the strip prior to corrugation to reinforce the edge and reduce the tendency to folding over under the Hot Shake Test.
Reference may be had to U.S. Pat. No. 4,381,590 dated May 3, 1983 to Nonnenmann et al which discloses a spirally wound monolith made up of corrugated and flat continuous strips which are brazed together. No collimator device such as here employed, is disclosed. However, the basic regular uniform pitch corrugated foil strip backed up with a flat strip and spirally wound, as described therein, may be used herein.
Reference may also be had to U.S. Pat. No. 3,768,982 to Kitzner dated Oct. 30, 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to a desired temperature. Reference may also be had to U.S. Pat. No. 3,770,389 to Kitzner dated Oct. 30, 1973 which discloses a central electrically heated core within a ceramic monolith, heat being transmitted by conduction to the catalyst contained in the openings of the ceramic monolith. The heating core is formed of metal sheets, one corrugated, the other flat, coated with alumina and bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance. Heating of the ceramic core by conduction takes too long to solve the problem of atmospheric pollution at start-up. The metallic cores are subject to telescoping under the conditions of the Hot Shake test.
The Hot Shake test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at high temperature (between 700 and 950 C.; 1292 to 1742 F., respectively) with exhaust gas from a running internal combustion engine being passed through the device. If the electrically heatable catalytic device telescopes in the direction of gas flow or breaks up after a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Usually, a test device will fail in 5 hours if it is going to fail. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
Reference may also be had to U.S. Pat. No. 4,711,009 to Cornelison et al dated Dec. 8, 1987 for details of a process for corrugating thin metal foil strips. Brazing cannot be satisfactorily done over a refractory metal oxide coated and catalyst bearing surface. Hence, in the present case, the refractory metal oxide/catalyst coating must be removed from those regions of the strip undergoing brazing or spot welding, for example, by wire brushing. The corrugating of a thin metal foil strip with a herringbone or chevron pattern as taught therein is applicable to the present invention and hence, the disclosure of the aforesaid U.S. Pat. No. 4,711,009 is incorporated herein by reference. Alumina wash coating and catalyst application, using materials disclosed in U.S. Pat. No. 4,711,009, may be done after brazing by a dipping process, if desired.
Ordinarily the thickness of the foil strip has been in the range of from 0.001" to 0.005" with a preference for 0.0015". It has been found that increasing the preferred thickness up to 0.0016" to 0.0019" aids in reducing the tendency to fold over under the Hot Shake Test. The corrugations generally have an amplitude of from 0.02" to 0.25" and a pitch of from 0.02" to 0.2". The cross-sectional shape of the corrugations may be triangular, truncated triangular, triangular with the apices rounded, wave-like, e.g., sinusoidal, etc. The pattern of corrugation may be herringbone or chevron with a slope of from 3 degrees to 10 degrees to a line perpendicular to the edges of the foil strip, or they may be straight through according to a variable pitch pattern (Bullock et al, supra) or if, the foil is to be backed up with a flat foil strip, a uniform straight-through pattern with the corrugations lying along lines perpendicular to the longitudinal edges of the foil strip. Generally the pitches at the higher end of the range, e.g., 0.1 to 0.15" are desired in the present case.
Reference may also be had to International PCT publication numbers WO 89/10471 and WO 89/10470 each filed Nov. 2, 1989. S-wound cores composed of corrugated and flat strips in alternating relation are disclosed in these publications. However, there is no teaching of the use of a collimator device therein, and telescoping of the core can occur under the conditions of the test described above.
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for this alloy is described in U.S. Pat. No. 4,414,023 to Aggen dated Nov. 8, 1983. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more thereof, balance iron and trace steel making impurities.
In the following description, reference will also be made to fibrous ceramic mat or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Sowman for formulations and manufacture of ceramic fibers and mats useful herein. One such ceramic fiber material is currently commercially available from 3-M under the registered trademark "INTERAM."
Electrically heatable catalytic converters of the spirally wound type exhibit two types of mechanical failure when subjected to automobile durability tests, such as, the "hot shake" test. Telescoping failure can be largely eliminated by proper design of the monolith, such as shown, for example, in commonly owned copending application Ser. No. 626,672 filed Dec. 12, 1990, the disclosure of which is incorporated herein by reference. The second type of failure, which is called "face separation," is a condition where face portions of successive convolutions of a spirally wound metal monolith bend over and/or part under the influence of high speed gas flow and tend to obstruct gas flow. This condition is not acceptable.
It has been found that by positioning a protective element just upstream, i.e., axially within from 0.1 inch plus or minus 0.05 inch to 1.5 inch plus or minus 0.2" hereinafter "from about 0.1 inch to about 1.5 inch," face separation can be eliminated. The protective element straightens out and evens the fluid flow pattern presented to the face of the electrically heatable catalytic converter, hereinafter "EHC," so that only axial forces act on the EHC. Thus, bending or folding of the leading edges of the foil convolutions is avoided.